A hybrid electric vehicle alternates between gas-powered engine operation and electric motor operation. The transition from operating the vehicle in a gas-powered mode to operating the vehicle in an electric mode, and vice versa, occurs while the vehicle is being used. Therefore, an effective transition from operation in one mode to another is imperative so that no loss of vehicle function or control occurs during the vehicle operation. For example, when transitioning from gas-powered operation to electric mode operation, care must be taken to ensure that the vehicle's electric mode operation is fully functional before shutting down the vehicle's gas-powered engine.
Currently, however, the potential for ineffective transitioning between operating modes exists. Specifically, problems may occur in transitioning between use of the gas-powered engine to pump transmission fluid and an auxiliary transmission fluid pump in the electric mode. In an advanced hybrid system truck (“AHS-T”) hybrid transmission, an auxiliary pump provides pressurization of transmission fluid when the vehicle is in electric mode. The main pump driven by the gas-powered engine itself provides transmission fluid pressurization when the vehicle is in a gas-powered mode. During the transition from gas-powered engine propulsion to electric propulsion, the auxiliary pump is activated and the gas-powered engine (including the main pump) is allowed to shut down. Unfortunately, conventional hybrid transmissions are susceptible to prematurely shutting down the gas-powered engine before the auxiliary pump is fully operational. Specifically, during cold ambient temperatures or cold transmission fluid temperature conditions, the auxiliary pump is prone to stall during its initial activation stage. When this happens, the vehicle may lose power and control. The consequences of the power failure range from simple inconvenience to unacceptably dangerous. Therefore, systems and methods that ensure a proper transition from gas-powered engine operation to electric motor operation are clearly desirable.